1. Field of the Invention
The present invention relates to an optical transmitter-receiver module having a three-dimensional structure in which a light-emitting device and a light-receiving device are housed together in a metallic package. The present invention intends to offer a device structure that reduces optical crosstalk between the light-emitting device and the light-receiving device. Here, the term “crosstalk” is used to mean that part of the signal of the light-emitting device intrudes into the light-receiving device in the same node. The crosstalk is classified into electrical crosstalk, electromagnetic crosstalk, and optical crosstalk. Here, the present invention tackles a problem of optical crosstalk that causes noise due to the intrusion of the lightwave of the light-emitting device into the light-receiving device.
2. Description of the Background Art
At present, a metallic can package-type optical transmitter-receiver module has the following structure. A light-receiving device is housed in a metallic can package, and a light-emitting device is housed in another metallic can package. Two optical fibers connected to the individual packages are connected to a wavelength-demultiplexing-and-multiplexing device (WDM). The WDM is further connected to an optical fiber. In this case, the light-receiving device (PD, APD) and the light-emitting device (LD, LED) are housed in the independent packages and connected through an optical fiber. Therefore, there is no possibility of occurring optical crosstalk.
A metallic can package type having a three-dimensional structure has been developed. This package has the following structure. A wavelength-selecting filter is provided right before an optical fiber. The outgoing signal lightwave of a light-emitting device passes through the wavelength-selecting filter in a straight line to enter the optical fiber. The incoming signal lightwave having travelled over the optical fiber is reflected from the wavelength-selecting filter to enter the light-receiving device. This three-dimensional structure houses the light-receiving device and the light-emitting device in a metallic can package. The reason this type is developed is that it has excellent air-tightness and therefore high reliability.
In this case, if a surface extending in a direction of the axis exists at the side of the wavelength-selecting filter, there is a possibility that a lightwave reflected from the wavelength-selecting filter is again reflected from that surface to enter the light-receiving device.
Such a possibility is explained below by referring to FIG. 1. An outgoing signal lightwave (solid line) generated by a light-emitting device (hereinafter simply referred to as LD) 3 is condensed by a lens 4, passes through a wavelength-selecting filter 5 obliquely, and arrives at the end of an optical fiber 6. The outgoing signal lightwave enters the optical fiber to travel over it. In front of the wavelength-selecting filter 5, a lens 7 and a light-receiving device (hereinafter simply referred to as PD) 8 are provided. An incoming signal lightwave (broken line) having travelled over the optical fiber 6 is reflected from the wavelength-selecting filter 5, bends its optical pathway about 90 degrees, is condensed by the lens 7, and enters the PD 8. With respect to the wavelength-selecting filter 5, at the opposite side of the PD 8, some wall 9 sometimes exists. In this case, a problem will be created.
The wavelength-selecting filter 5 is designed to transmit almost all, not completely all, of the lightwave of the LD, which enters the filter 5 at an angle of 45 degrees. The increase in the transmittance to a value close to 100% requires an increase in the number of layers in the dielectric multilayer film. This results in a cost increase. Consequently, a dielectric multilayer filter must be used in which the number of layers is limited to a certain extent. As a result, the filter cannot transmit all of the 45-degree-incident lightwave from the LD. Therefore, part of the outgoing signal lightwave from the LD 3 is reflected from the wavelength-selecting filter 5. In other words, part of the lightwave from the LD is reflected and bends its optical pathway about 90 degrees. The reflected lightwave hits the wall 9 to be reflected again directly. It returns the same pathway, arrives at the wavelength-selecting filter 5, and passes through it at a transmittance close to 100%. Subsequently, it passes through the lens 7 and arrives at the PD 8. In other words, when the wall 9 exists at the behind of the wavelength-selecting filter 5, part of the lightwave from the LD 3 enters the PD 8, causing optical crosstalk.
When crosstalk occurs between the light-emitting device and the light-receiving device in a module, the signal-to-noise ratio of the received signal lightwave decreases considerably. Such a module is not suitable as an optical module for optical communication. It is necessary to prevent the outgoing signal lightwave of the light-emitting device from entering the light-receiving device. Although having variations, the optical crosstalk amounts to −40 to −30 dB or so due to the imperfectness of the wavelength-selecting filter 5 and the existence of the wall 9. Depending on the purpose, it may be insufficient. For example, the crosstalk is sometimes required to be suppressed to −47 to −50 dB or below.
For example, it is assumed that an optical transmitter-receiver module as shown in FIG. 2 is produced. A package 20 of an LD comprises a cylindrical surface 23, a lens-supporting face 24, and a bottom face 25 and consequently has an internal space 22. An LD chip 3 is fixed in the internal space 22. Actually, a stem has a bump to which the LD chip 3 is fixed. Nevertheless, such an internal structure is omitted in FIG. 2. The LD chip 3, the package 20, and the like constitute the LD as a whole.
A package 30 of a PD comprises a cylindrical surface 33, a lens-supporting face 34, and a bottom face 35 and consequently has an internal space 32. A PD chip 8 is fixed in the internal space 32. The PD chip 8 and the package 30 constitute the PD as a whole. Although not shown, lead pins protrude from the back of the packages of the LD and PD.
A filter-holding sleeve 40 is provided to hold a wavelength-selecting filter 5. The filter-holding sleeve 40 has the shape of, for example, a rectangular solid. It comprises a bottom face 42, a left-hand-side face 43, an upper face 44, a right-hand-side face 45, and the like. The left-hand-side face 43 is also a side face of a left block 46. The right-hand-side face 45 is also a side face of a right block 47. A slanted face 48 extends upward obliquely from the left block 46 and connects to a slanted face 49 of the right block 47. The slant angle Ψ of the slanted faces 48 and 49 is close to 45 degrees. Nevertheless, it may deviate from the 45 degrees to a small extent. It is essential only that the position of the PD be properly determined by the slant angle Ψ. An opening 50 is provided between the slanted faces 48 and 49, and the wavelength-selecting filter 5 is fixed there. The wavelength-selecting filter 5 is a dielectric multilayer filter designed such that when the incoming signal lightwave (e.g., 1.55 μm) enters it at an angle of 45 degrees, it reflects the lightwave, and when the outgoing signal lightwave (e.g., 1.3 μm) enters it at an angle of 45 degrees, it transmits the lightwave.
The end portion of an optical fiber 6 is fixed at an opening 52 at the center of the upper face 44 of the sleeve 40. Another opening 53 is provided at the upper portion of the left-hand side face 43 of the sleeve 40 to fix the package 30 of the PD. Thus, the LD and PD housed in the package are fixed to the filter-holding sleeve 40. The optical axis of the optical fiber lies on an extension of the optical axis of the LD. When the optical axis of the PD is shifted line-symmetrically with respect to the wavelength-selecting filter 5 as the center, the shifted optical axis lies on an extension of the optical axis of the optical fiber.
The sleeve 40 holding the wavelength-selecting filter 5 has solid blocks 46 and 47 at its bottom half. The inner side face of the right-hand block 47 forms a wall 9 facing the wavelength-selecting filter 5 at a short distance.
Most of the outgoing signal lightwave emitted from the LD 3 passes through the wavelength-selecting filter 5, advances in a straight line, and enters the optical fiber 6. However, it is difficult to produce the wavelength-selecting filter 5 having a 100% transmittance for the lightwave of the LD. Because of the imperfectness of the wavelength-selecting filter 5, part of the lightwave of the LD is reflected and arrives at the wall 9. The wall 9 has a smooth face and reflects the lightwave of the LD to the opposite direction. Almost 100% of the reflected lightwave passes through the wavelength-selecting filter 5. Then, the lightwave is condensed by a lens 7, and enters the PD 8. In other words, part of the outgoing signal lightwave of the LD 3 enters the PD 8. The reflection from the wall 9 is close to 100%, and the lightwave passes through the wavelength-selecting filter 5 almost 100%. Therefore, the power of the outgoing signal lightwave entering the PD depends almost on the performance of the wavelength-selecting filter 5. Such a structure creates a crosstalk of −30 to −40 dB.
If another wavelength filter (WDM) that reflects the wavelength of the outgoing signal lightwave and transmits the wavelength of the incoming signal lightwave is placed on an optical pathway 54 connecting the wavelength-selecting filter 5 and the lens 7, the crosstalk can be decreased. However, this structure further requires a high-cost WDM, increasing the cost of the module.
The present inventor found Patent literature 1 that raises a problem of crosstalk between the LD and PD. The optical module discussed in Patent literature 1 has a structure in which a PD is formed by housing a PD chip in a metallic can package, and an LD is formed by housing an LD chip in another metallic can package. The PD and LD are fixed on two side faces of a frame body for housing a wavelength-selecting filter. The PD and LD are coupled with a single optical fiber.
Patent literature 1: the published Japanese patent application Tokukai 2004-012647.
In Patent literature 1, the optical module has a structure in which an LD and a PD are coupled with an optical fiber through a wavelength-selecting filter. Patent literature 1 has proposed the following three methods: (a) a side wall of the filter-housing body is provided with a hole, (b) a side wall is formed with a face giving diffused reflection, and (c) a side wall is coated with frosting paint. According to the literature, although the lightwave from the LD is reflected from the wavelength-multiplexing-and-demultiplexing filter, the reflected lightwave passes through the through hole formed in the side wall of the filter-housing body to exit to the outside without entering the PD. The literature emphasizes that this structure should be able to eliminate the PD noise.
The literature states that when the side wall of the filter-housing body is formed with a face giving diffused reflection, the reflected lightwave from the LD is further reflected diffusedly there. Consequently, the lightwave arriving at the PD should be decreased, according to the literature. Alternatively, the literature describes when the side wall of the filter-housing body is coated with frosting paint, the reflected lightwave from the LD is absorbed and does not enter the PD. In short, the literature intends to decrease the reflected lightwave with the through hole, surface roughening, or painting.
All of the measures proposed by Patent literature 1 are imperfect. The idea of providing a through hole in the side wall in order to eliminate the reflection is not desirable because part of the outgoing signal lightwave leaks to the outside. The outgoing signal lightwave having exited to the outside from the through hole has a possibility of returning after being reflected again. In addition, stray lightwaves at the outside may enter the PD. The external environment may affect considerably. That is also undesirable.
Patent literature 1 proposes another method of applying frosting paint. However, it cannot be said that frosting paint absorbs the lightwave completely. Some of the lightwave may be reflected to enter the PD. The method must be said imperfect. Patent literature 1 proposes yet another method of roughening the side wall to reflect the lightwave diffusedly. The diffused reflection may sound a good idea. Nevertheless, some of the lightwave may enter the PD, and the chance of the entrance must be treated by probability. Therefore, the method must be said imperfect.